Solving High Voltage Current Sensing Design Challenges in HEV/EV

Electrification has created a new paradigm for automotive powertrains—whether the design is a hybrid electric vehicle (HEV) or an electric vehicle (EV), there are always new design challenges to solve. In this technical article, I want to highlight some of the main challenges of high voltage current sensing and share additional resources to help and simplify your design process.

For an introduction to current sensing, see our ebook “Current Sensing Simplified”

High Voltage, High Current: (>200 A or more commonly 1,000 A)

The high-voltage (≥400 V) all-electric system is designed to reduce the current consumption of the traction system that drives the vehicle. This requires an isolation solution so that the “hot” high voltage side can provide current measurements to the “cold” side (connected to a low voltage ≤5-V microcontroller or other circuit). Due to I2R power dissipation, high currents can be problematic when measured with shunt resistors.

Using a shunt in these cases means you have to choose a shunt resistor below 100-?Ω, but these tend to be larger and more expensive than the more common milliohm resistors. Another option is to use magnetic solutions, but these are less accurate and have higher temperature excursions than shunt-based solutions. If these performance deficiencies are overcome, the cost and complexity of magnetic solutions will be greatly increased.

Learn more with these design resources:

· “Design Considerations for Dual DRV425 Busbar Applications

· “Principle of bus operation

High voltage, low current (>400 V and <500 A)

Additionally, high voltages require an isolation solution. From a current standpoint, anything below 100 A is basically a shunt based solution. Between 100 A and 500 A, choosing a shunt versus a magnetic solution involves a trade-off between cost, performance, and solution size. The white paper describes:

“Comparing Shunt-Based and Hall-Based Current Sensing Solutions in On-Board Chargers and DC/DC Converters”

Precision measurement on 48-V rail, low current (<100 A)

The main design challenge for a 48-V rail is the survivability voltage required to meet your requirements, which can be as high as 120 V. In some 48-V motor systems, high-accuracy current measurement is required to peak motor efficiency. These motor systems may include traction inverters, electric power steering, or generators with starters. Online measurements can show the most accurate actual motor current, but are also very challenging due to the presence of high-speed pulse width modulation (PWM) signals, as described below:

“Low-Drift, High-Accuracy, In-Line Motor Current Measurement with Enhanced PWM Rejection”

For non-motor 48-V systems, such as DC/DC converters or battery management systems (BMS), achieving bidirectional DC current measurement is more critical than achieving switching performance, as described below:

“High Side Bidirectional Current Sensing Circuit with Transient Protection”

Eliminates high-voltage common-mode voltage requirements for low-side induction

Low-side current sensing reduces the requirements of some amplifiers: the input does not need to experience high voltages because the common mode of low-side sensing is ground -0 V.

The common-mode voltage range of the amplifier must include 0 V for measurement on the low side. If the application is motor low-side phase current measurement, the amplifier must have a very high slew rate to adjust the switching on and off, as described below:

“Low-drift, low-side current measurement for three-phase systems”

For non-motor applications, your choice depends on the accuracy requirements of the implementation. See:

“Low-side current detection circuit integration”

“External Current Sense Amplifier and Integrated On-Board Amplifier for Current Sensing”

Measuring multi-segment currents in BMS

High-accuracy, multi-segment current measurement (from mA to 1kA) is a significant challenge to solve in a single solution. Magnetic solutions do not measure low currents well because of their higher offset levels and significant drift. Due to the extremely low differential input voltage levels, shunt-based measurements require very low offset to be able to measure low currents on sub-shunt resistors below 100-μΩ.

For example, a BMS might want to measure ±1,500 A. For bidirectional measurements of 0-A output voltage and ±2.5-V output swing with a gain of 20, the maximum input voltage is ±125 mV. This results in a shunt resistor value of ≤ 83 Ω. The voltage drop across this shunt is only 8.3V at 100mA, which means you need an amplifier system with very low offset to measure this level. If the offset of the system is 1 V, this level error is ~16%.

To learn more, read:

“Shunt-Based Current Sensing Solutions for BMS Applications in HEVs and EVs”

Current sensing in solenoids for smoother actuation

Many automotive applications use proportional solenoids, but for high voltage current sensing, proportional solenoids are primarily used in automatic transmissions. Proportional solenoid valves provide a smooth ride when shifting gears or running the hydraulic pump. The actuation capability of a solenoid valve mainly depends on two factors: solenoid valve actuation and solenoid valve position sensing.

High-precision current measurement enables precise closed-loop control of the electromagnetic plunger position.

Current sensors in solenoid valve applications follow the shunt principle. A pulse width modulated signal can be flowed across the solenoid valve through a milliohm shunt. This milliohm shunt is integrated inside or outside the current sense amplifier, depending on the current range.

For more details on solenoid valve current sensors, check out:

“Current Induction Dynamics in Automotive Solenoid Valves.”

“Automotive Proportional Solenoid Valve with High Accuracy Current Sensor Reference Design”

“Reference Design for Automotive Proportional Solenoid Current Sensor”

Current sensing is a fundamental element of increased electrification in automotive design, especially in high-voltage systems. While modern cars are more demanding than ever for sensors, the resources I have linked in this article can help you design a powertrain that performs well and delivers safe power.

The Links:   NL8060BC31-02 FP15R12NT3


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